1. Field of the Invention
The present invention relates to nail guns having a contact arm vertically movable relative to a nail gun body and a device that can prevent the movement of the contact arm toward the upper stroke end position when the number of nails stored in a magazine is less than a predetermined number.
2. Description of the Related Art
A known nail gun has a contact arm that is vertically movable along a driver guide. The drive guide extends downwardly from a nail gun body and has a nail discharge opening at its lower end position. The contact arm is normally held at a lower stroke end position, in which the contact arm extends downward from the lower end position of the driver guide and prevents a trigger from operating to drive nails. When an operator presses the nail gun against a workpiece, the contact arm may contact the workpiece before the driver guide contacts the workpiece. As a result, the contact arm moves upward along the driver guide. When the lower end position of the contact arm contacts the workpiece, the contact arm reaches the upper stroke end position, in which the trigger is permitted to operate for driving the nails.
On the other hand, a pusher is associated with the magazine for feeding the nails one after another into the driver guide. An "idle driving prevention device" typically comprises an engaging member that is connected to the pusher. An "idle driving prevention device" means a mechanism that prevents the nail gun from driving nails when the magazine is empty or nearly empty. The pusher moves toward the driver guide as the number of nails stored in the magazine is reduced. When the number of nails stored in the magazine is reduced to a predetermined number, the engaging member enters the moving path of the contact arm to prevent the contact arm from moving upward from its lower stroke end position. As a result, the nail driving operation can be prevented when the magazine is empty or nearly empty.
However, the engaging member of the idle driving prevention device moves to enter the moving path of the contact arm with the pusher instantaneously with the driving operation of the nail that has been fed immediately before the predetermined number has reached. At that moment, the contact arm is positioned at the upper stroke end position or a position slightly below the upper stroke end position. Therefore, the engaging member may abut the contact arm. As a result, the engaging member may interfere with the contact arm and lock the same in the position above the lower stroke end position. When this lockup occurs, the contact arm may not return to the lower stroke end position and therefore, the nail driving operation cannot be smoothly performed.
The above situation could possibly be caused every time in which a nail is driven that is immediately before the predetermined last nail in the magazine (which may be zero nails). Therefore, the above situation is a significant problem to be solved.
In order to ensure that the contact arm may return to the lower stroke end position against the interference with the engaging member, it is possible to increase the biasing force of the spring that biases the contact arm. However, if the biasing force of the spring has been increased, a great pressing force is required to move the contact arm from the lower stroke end position to the upper stroke end position. As a result, another problem may be caused, because the operability of the nail gun is more cumbersome.
Another significant problem with known nail guns occurs when several nails are driven into the same workpiece with the driving position in turn shifted to a different positions adjacent thereto. In this situation, the operator may frequently perform a "dragging" driving operation in the following manner:
After one nail driving operation has been completed, the operator shifts the nail gun to the next driving position with the contact arm held in contacted with the workpiece or at least the contact arm is not permitted to completely return to the lower stroke end position. When the nail gun reaches the next driving position, the operator continues to press the nail gun to move the contact arm to the upper most position, so that the operator can drive the next nail by the operation of the trigger.
If the driving operation of the nail is continuously performed several times and the contact arm has not been completely returned to the lower stroke end position as described above, it is inevitable that the engaging member will abut the contact arm and lockup the same. Thus, because the engaging member enters the moving path of the contact arm when the number of the nails in the magazine has been reduced to a predetermined number, the contact arm may still be locked by the engaging member even if the previous problem has been solved.